This invention relates generally to the recording of digital signals and pertains more specifically to a method of, and apparatus for, recording a super-audio or like digital signal in which each sample of the original analog signal is expressed by one bit of digital data, that is, by either binary 0 or 1.
The so called “super audio compact disk” (SACD), with its greatly extended range of frequencies compared to that of the more conventional compact disk, has recently been developed by SONY® and PHILIPS® and introduced to the market the world over. Recorded on the SACD is what is known as the “direct stream digital” (DSD) signal. The DSD is essentially equivalent to delta-sigma (or sigma-delta) modulation, in which each sample of the analog signal is translated into either of the two binary digits, 0 and 1, as will later be explained in more detail. Japanese Unexamined Patent Publication No. 2000-114971 is hereby cited as dealing with delta-sigma modulation.
Whereas each sample of the audio signal is translated into a plurality of, sixteen for example, bits in the more conventional pulse-code modulation, the delta-sigma modulation employs but either of the binary digits 0 and 1 for expressing each sample of the analog signal. Let it be assumed that a stream of audio signal is now both delta-sigma and pulse-code modulated into two different digital signals that are the same in the total number of bits. Then the sampling frequency of delta-sigma modulation can be made very much, sixteen times for example, higher than that of pulse-code modulation. A higher-resolution analog-to-digital conversion is therefore possible by delta-sigma modulation without an increase in the total number of bits for a given signal volume.
Being inherently different from pulse code modulation, delta-sigma modulation has so far demanded a dedicated recorder. As currently manufactured, such dedicated delta-sigma modulation recorders are very expensive because they are not currently mass-produced by reason of the yet limited popularity of SACDs.